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db io Appendix A: Description of the Environmental Fluid Dynamics Code A.1 Nearshore Wave -Induced Currents and Sediment Transport Extensions EFDC includes a number of extensions for simulation of nea.rshore wave -induced currents and noncohesive sediment transport (Zarillo and Surak, 1995). The extensions include: a wave -current boundary layer formulation similar to that of Grant and Madsen (1986); modifications of the hydrodynamic model's momentum equations to renrecPnt wave, period averaged tulenan mean quantities; the inclusion of the three-dimensional wave induced radiation or Reynold's stresses in the momentum equations, and modifications of the velocity fields in the transport equations to include advective transport by the wave induced Stoke's drift. High frequency surface wave fields are provided by an external wave refraction -diffraction (Zarillo and Surak, 1996). A.2 Hydrodynamics, Salinity, and Temperature Transport The physics of the EFDC model and many aspects of the computational scheme are equivalent to the widely used Blumberg- Mellor model (Blumberg & Mellor, 1987) and U. S. Army Corps of Engineers' CH3D or Chesapeake Bay model (Johnson, et al, 1993). The EFDC model solves the three-dimensional, vertically hydrostatic, free surface, turbulent averaged equations of motions for a variable density fluid. Dynamically coupled transport equations for turbulent kinetic energy, turbulent length scale, salinity and temperature are also solved. The two turbulence parameter transport equations implement the Mellor-Yamda level 2.5 turbulence closure scheme (Mellor & Yamada, 1982; Galperin et al, 1988). The EFDC model uses a stretched or sigma vertical coordinate and Cartesian, or curvilinear orthogonal horizontal coordinates. The numerical scheme employed in EFDC to solve the equations of motion uses second order accurate spatial finite differencing on a staggered or C grid. The model's time integration employs a second order accurate three -time level, finite difference scheme with an internal-external mode splitting procedure to separate the internal shear, or baroclinic mode, from the external free surface gravity wave, or barotropic mode. The external mode solution is semi -implicit, and simultaneously computes the two-dimensional surface elevation field by a preconditioned conjugate gradient procedure. The external tnhitinn is competed by the calculation of the depth average barotropic velocities using the new surface elevation field. The model's semi -implicit external solution allows large time steps which are constrained only by the stability criteria of the explicit central difference or high order upwind advection scheme (Smolarkiewicz and Margolin, 1993) used for the nonlinear accelerations. Horizontal boundary conditions for the external mode solution include options for simultaneously specifying the surface elevation only, the characteristic of an incoming wave (Bennett & McIntosh, 1982), free radiation of an outgoing wave (Bennett, 1976; Blumberg & Kantha, 1985) or the normal volumetric flux on arbitrary portions of the boundary. The EFDC model's internal momentum equation solution, at the same time